Ultraviolet (UV) light affects the human body in both beneficial (e.g., vitamin D and tanning) and detrimental (e.g., skin wrinkling, skin cancer and sun burn or erythema) ways. UV light is typically more difficult to measure than visible and near infrared light because the spectral content is much weaker than visible light and the short wavelength provides an abundance of challenges for UV sensor systems.
The UV spectrum is made up of three regions: UVA, UVB and UVC. Solar UVC radiation is blocked by the earth's atmosphere. Solar UVB light is partially blocked by the stratospheric ozone layer, and UVA light largely transmits. Both UVA and UVB light experience significant Rayleigh scattering—the phenomenon responsible for making the sky blue. The UVB spectral range (˜280-315 nm) includes shorter wavelengths than the UVA spectral range (˜315-400 nm) and is mostly responsible for sunburn, carcinoma of the skin and vitamin D generation. UVA includes longer wavelengths that cause tanning, freckles and skin aging effects.
For effective UV index calculation, UV sensors must be capable of estimating the solar spectrum incident at ground level accurately. The solar spectrum varies with environmental variables such as zenith angle, atmospheric ozone concentration, altitude and cloud cover. Existing UV sensors techniques typically use a photodiode optimized for detecting wavelengths in the 280 nm to 400 nm range, manufactured in process technologies such as fully depleted (thin film) silicon on insulator technology, Gallium Nitride or Silicon Carbide technologies. In most cases, additional filtering is applied to further improve the selectivity for UV wavelengths. These filters can be applied directly on the sensor, often using wafer level processing techniques, and/or integrated into the housing of the final product or device.